X9420

FN8195 Rev.1.00

Page 4 of 19

April 26, 2006

PIN NAMES

PRINCIPLES OF OPERATION

The

X9420

is

a

highly

integrated

microcircuit

incorporating a resistor array and associated registers

and counter and the serial interface logic providing direct

communication between the host and the XDCP

potentiometer.

Serial Interface

The X9420 supports the SPI interface hardware

conventions. The device is accessed via the SI input

with data clocked in on the rising SCK. CS must be LOW

and the HOLD and WP pins must be HIGH during the

entire operation.

The SO and SI pins can be connected together, since

they have three state outputs. This can help to reduce

system pin count.

Array Description

The X9420 is comprised of one resistor array containing

63 discrete resistive segments that are connected in

series. The physical ends of each array are equivalent to

the fixed terminals of a mechanical potentiometer

At both ends of the array and between each resistor

segment is a CMOS switch connected to the wiper

switch may be turned on at a time.

These switches are controlled by a Wiper Counter Register

(WCR). The six bits of the WCR are decoded to select, and

enable, one of sixty-four switches. The block diagram of the

potentiometer is shown in Figure 1.

Wiper Counter Register (WCR)

The X9420 contains a Wiper Counter Register. The

WCR can be envisioned as a 6-bit parallel and serial

load counter with its outputs decoded to select one of

sixty-four switches along its resistor array. The contents

of the WCR can be altered in four ways: it may be written

directly by the host via the Write Wiper Counter Register

instruction (serial load); it may be written indirectly by

transferring the contents of one of four associated data

registers via the XFR Data Register instruction (parallel

load); it can be modified one step at a time by the

Increment/ Decrement instruction. Finally, it is loaded

with the contents of its data register zero (DR0) upon

power-up.

The Wiper Counter Register is a volatile register; that is,

its contents are lost when the X9420 is powered-down.

Although the register is automatically loaded with the

value in DR0 upon power-up, this may be different from

the value present at power-down.

Data Registers

The potentiometer has four 6-bit nonvolatile Data Registers.

These can be read or written directly by the host. Data can

also be transferred between any of the four Data Registers

and the WCR. It should be noted all operations changing

data in one of the Data Registers is a nonvolatile operation

and will take a maximum of 10ms.

If the application does not require storage of multiple

settings for the potentiometer, the Data Registers can be

used as regular memory locations for system

parameters or user preference data.

Register Descriptions

Table 1. Data Registers, (6-bit), Nonvolatile

There are four 6-bit Data Registers associated with the

potentiometer.

– {D5~D0}: These bits are for general purpose Nonvola-

tile data storage or for storage of up to four different

wiper values.

Table 2. Wiper Counter Register, (6-bit), Volatile

– {WP5~WP0}: These bits specify the wiper position of

the potentiometer.

Symbol

Description

SCK

Serial Clock

SI, SO

Serial Data

A0

Device Address

Potentiometer Pins (terminal equivalent)

Potentiometer Pins (wiper equivalent)

WP

Hardware Write Protection

HOLD

Serial Communication Pause

V+,V-

Analog Supplies

System Supply Voltage

System Ground

NC

No Connection

0

0

D5

D4

D3

D2

D1

D0

(MSB)

(LSB)

0

0

WP5 WP4 WP3 WP2 WP1 WP0

(MSB)

(LSB)